+1 on low dissolved oxygen.

In my old aquarium keeping days, a 10% water change was considered safe so long as the frequency was often enough. The frequency depends on the bio load in the aquarium. If I let the tank get off schedule, I would often perform a 20% water change without issue. I was not pushing the limits of biomass to gallons of water.

How much water was exchanged? How often is this performed? How much biomass in the tank? What was the difference in water temp between tank water and new water? What's the average length of fish?

Fish on!,
Noel